The present invention is generally directed toward an improved rigid mounting for a mirror or plurality of mirrors which are used in a scanning optical system, and more particularly, to a mirror structure design which is especially useful in a laser printing system.
Printing system utilizing lasers to reproduce information are well known in the art. Typically, the system includes a laser for generating a laser beam, a modulator, such as an acousto-optic modulator, for modulating the laser beam in accordance with input information to be reproduced, a multi-faceted scanner for scanning the modulated laser beam across a medium on a line to line basis, and various optical components to focus and align the laser beam onto the medium. The Xerox 4045 printer is an example of this type of printing system.
A compact design for the scanning optics of these prior art type of printers is desirable to make the machine itself as compact as possible. The scanning system is usually reduced in total area by folding the beam path by the use of a plurality of mirrors. An example of a compact modular laser printing system is shown in U.S. Pat. No. 4,230,902. As shown in FIGS. 2 and 3 of that patent, the output laser beam 60 is directed through modulator 62 and folded by mirrors 62, 64, and 66. The aligning of these mirrors is a critical factor in the performance of the scan system, since any deviation from the required angular orientation would result in nonuniformity at the scanned raster lines of the recording medium. The mounting structure for these mirrors was not disclosed in the '902 patent, but FIG. 1 of the present application shows a typical prior art mounting structure for mirrors mounted in a raster output scan (ROS) printing system. A mirror 10 is mounted in a recess shelf portion of a first, relatively narrow arm 12. Arm 12 has a wider area 14 having a threaded aperture 16 therethrough. Arm 12 is connected to, a base portion 18 which is fixedly secured to the ROS housing. A relatively thick, upwardly extending arm 20 is also connected to and extends upwardly from, base assembly 18. Arm 20 has two apertures bored therethrough. Aperture 22 accommodates a bolt 26 having a threaded end which screws into threaded aperture 16. Set screw 28 is inserted through threaded aperture 24 until it comes into contact with arm 12. Vertical alignment of mirror 10 is accomplished by essentially providing two forces to arm 12. Arm 12 is of a material, (typically aluminum or steel) and thin enough to have a small degree of flexibility. A first "pull" force is applied via bolt 26 seated in aperture 27 through threaded aperture 16 which "pulls" arm 12 in the direction of arrow A. A second "push" force is applied by turning screw 28 so that it applies a force represented by arrow B against arm 12. In the usual alignment, screw 28 is turned 1/2 turn past the desired alignment point and the arm is pulled into arrow A's direction by means of the "pull" bolt 26.
One problem with this prior art design is that of mirror "thermal drifting" because of temperature changes. The metal structure of the mirror mounting assembly is subject to a slight degree of thermal expansion and contraction with temperature. The opposing forces (A and B) at different points of the arm holding the mirror create an inherently unequal stress within that mirror mounting assembly. During temperature changes, the original moment established by the two opposing forces aligning the mirror in the desired plane is slightly changed causing a mirror orientation "shift". This shift, or thermal drift results in beam deflection paths which deviates from the desired path. As a practical matter, this shift creates a mirror misalignment under two conditions. The first can be characterized as a thermal drift error due to the effects of hysteresis. For example, a ROS unit having multiple mirror structures may be aligned to the required precision at an ambient temperature at a test facility. The unit would then be shipped to the facility where it is to be operated. If the unit experiences temperature variations during transit, the mirror structure will undergo a permanent change in orientation when returned to the ambient temperature due to the effects of hysteresis. The second condition is the thermal drift actually experienced during temperature changes when being operated. A one, or two, or three mirror system may be tolerant enough to absorb a slight degree of thermal drift, but systems with 4, 5 or event 6 mirrors (such as the ROS unit used in the Xerox 8836 printer) cannot tolerate the amount of cumulative thermal drift resulting from the present mirror mounting designs.
The present invention is, therefore, directed toward an improved mirror mounting design which reduces the effects of thermal drift and hysteresis on the mirrors used in a ROS-type printing system. More particularly, the invention is directed towards a mirror mounting assembly comprising a mounting structure having a base area adapted to be secured to a fixed location, said base area having a relatively thicker arm portion projecting away therefrom, said arm portion connected to a mirror mount portion at its upper end by a relatively narrow, slightly flexible neck portion, and means for applying a pushing force against at least two vertical points of said mirror mount portion.